The development of a novel SiC crystal growth technique, generically described as High Temperature Chemical Vapor Deposition (HTCVD) is reviewed. The structural, optical and electrical properties of 4H-SiC semi-insulating substrates are investigated with the aim of providing optimal microwave device performances. In particular, alternative compensation mechanisms to vanadium doping in S.I substrates are investigated to eliminate substrate induced trapping effects. Carried out at temperatures above 2100°C, the HTCVD technique uses, as in CVD, gas precursors (silane and a hydrocarbon) as source materials. The growth process can be described as “Gas Fed Sublimation” and proceeds by the gas phase nucleation of Six-Cy clusters, followed by their sublimation into active species that are condensed on a seed. Crystals with diameters up to 45 mm have been obtained with growth rates of 0.6 mm/h. The use of specific process steps, such as in-situ seed surface preparation and micropipe closing are presented and high resistivity wafers with micropipe densities down to 10 cm−2 are demonstrated. 4H-SiC substrates prepared from undoped crystals (with vanadium concentration lower than 5×1014 cm−3) exhibit semi-insulating behavior with a room temperature resistivity of the order of 1010Ω?cm. Infrared absorption measurements show that two types of semi-insulating crystals can be grown, with a spectrum either dominated by the Si-vacancy, or by a previously unreported defect labeled UD-1. These two types of semi-insulating wafers are also differentiated by the temperature dependence of their resistivity, with activation energies of 0.85 and 1.4±0.1 eV, respectively, and by the stability of their resistivity upon an annealing at 1600°C. Initial MESFET devices processed on HTCVD grown substrates show better DC characteristics than devices processed on vanadium doped substrates.